Abstract
Absorption, fluorescence, and fluorescence excitation spectra were studied for a number of
cation-anionic and related anionic polymethine dyes in weakly polarand nonpolarsolvents, as well as
in binary mixtures of solvents of different polarity. For some dyes, aggregation is observed in toluene or
acetonitrile-toluene mixtures with low amounts of acetonitrile, which is revealed as appearance of new absorption
bands and/or broadening of the initial bands of a monomeric dye. Solvent mixtures butyronitrilehexane
with low butyronitrile content were found to greatly stimulate the formation of dye aggregates for
most of the dyes studied. The absorption spectra of the aggregates are often blue-shifted with respect
to the corresponding absorption spectra of parent monomeric dyes and/or represent broad continuums
located both in the blue and red regions. For one of the cation-anionic dyes studied, which consists of